Mechanism of intergranular penetration of gallium in 7050 aluminum alloy

Liquid Metal Embrittlement (LME) is a phenomenon in which the ductility of a normally ductile solid metal is significantly reduced when it is brought into contact with certain liquid metals. Many models have been proposed in the past few decades to understand the mechanism of LME, but none of them can explain the complexity occurred in LME. The Al/Ga system is a remarkable example of intergranular penetration in that penetration can happen at room temperature without load applied. The Al/Ga system shows many common characteristics observed in LME including no intermetallic compound formed between the two elements, very limited mutual solubility, and small positive heat of mixing. The driving force for gallium penetrating aluminum grain boundaries is energy reduction when an aluminum grain boundary is replaced by two fresh surfaces covered with liquid gallium.; The investigation in this dissertation employed both experimental and theoretical approaches. The gallium penetration was performed at different temperature with no force applied to obtain the activation energy of this process in 7050 Al alloy with two heat treatment conditions: T74 and T7451. Crack growth tests were also performed to study the dependence of penetration rate on stress intensity factor. Penetrated grain boundaries were examined with SEM to determine the failure modes. It was found that the activation energy for gallium penetrating aluminum alloys was much smaller than that for solid metal diffusion. The crack growth rate depends on stress intensity factor exponentially. Slip bands were found on the gallium penetrated grain boundary surface even though there was no applied force.; Molecular Dynamics simulation was implemented to investigate the interaction of aluminum and a virtual liquid metal with lower melting point. The potential for virtual liquid metal was developed through manipulating the EAM potential for aluminum. The cross potential between aluminum and virtual liquid metal was developed in a similar way with considering the heat of mixing in Al/Ga system. Surface energy of clean and modified aluminum surface was calculated. The structure of the virtual liquid metal on aluminum surface, as well as confined between two aluminum walls, was studied with density and energy distribution function. The simulation suggested that a small positive heat of mixing between aluminum and virtual liquid metal is favorable for grain boundary penetration.